Traditional (non-lithium containing) aluminum alloys have been semi-continuously cast in open bottomed molds since the invention of Direct Chill (“DC”) casting in the 1938 by the Aluminum Company of America (now Alcoa). Many modifications and alterations to the process have occurred since then, but the basic process and apparatus remain similar. Those skilled in the art of aluminum ingot casting will understand that new innovations improve the process, while maintaining its general functions.
U.S. Pat. No. 4,651,804 describes a more modern aluminum casting pit design. It has become standard practice to mount the metal melting furnace slightly above ground level with the casting mould at, or near to, ground level and the cast ingot is lowered into a water containing pit as the casting operation proceeds. Cooling water from the direct chill flows into the pit and is continuously removed there-from while leaving a permanent deep pool of water within the pit. This process remains in current use and, throughout the world, probably in excess of 5 million tons of aluminum and its alloys are produced annually by this method.
Unfortunately, there is inherent risk from a “bleed-out” or “run-out” using such systems. A “bleed out” or “run out” occurs where the aluminum ingot being cast is not properly solidified in the casting mold, and is allowed to leave the mold unexpectedly and prematurely while in a liquid state. Molten aluminum in contact with water during a “bleed-out” or “run-out” can cause an explosion from (1) conversion of water to steam from the thermal mass of the aluminum heating the water to >212° F. or (2) the chemical reaction of the molten metal with the water resulting in release of energy causing an explosive chemical reaction.
There have been many explosions throughout the world when “bleed outs” “run-outs” have occurred in which molten metal escaped from the sides of the ingot emerging from the mold and/or from the confines of the mold, using this process. In consequence, considerable experimental work has been carried out to establish the safest possible conditions for DC casting. Among the earliest and perhaps the best known work was undertaken by G. Long of the Aluminum Company of America (“Metal Progress” May 1957 pages 107 to 112) (hereinafter referred to as “Long”) that was followed by further investigations and the establishment of industry “codes of practice” designed to minimize the risk of explosion. These codes are generally followed by foundries throughout the world. The codes are broadly based upon Long's work and usually require that: (1) the depth of water permanently maintained in the pit should be at least three feet; (2) the level of water within the pit should be at least 10 feet below the mold; and (3) the casting machine and pit surfaces should be clean, rust free and coated with proven organic material.
In his experiments, Long found that with a pool of water in the pit having a depth of two inches or less, very violent explosions did not occur. However, instead, lesser explosions took place sufficient to discharge molten metal from the pit and distribute this molten metal in a hazardous manner externally of the pit. Accordingly the codes of practice, as stated above, require that a pool of water having a depth of at least three feet is permanently maintained in the pit. Long had drawn the conclusion that certain requirements must be met if an aluminum/water explosion is to occur. Among these was that a triggering action of some kind must take place on the bottom surface of the pit when it is covered by molten metal and he suggested that this trigger is a minor explosion due to the sudden conversion to steam of a very thin layer of water trapped below the incoming metal. When grease, oil or paint is on the pit bottom an explosion is prevented because the thin layer of water necessary for a triggering explosion is not trapped beneath the molten metal in the same manner as with an uncoated surface.
In practice, the recommended depth of at least three feet of water is generally employed for vertical DC casting and in some foundries (notably in continental European countries) the water level is brought very close to the underside of the mold in contrast to recommendation (2) above. Thus the aluminum industry, casting by the DC method, has opted for the safety of a deep pool of water permanently maintained in the pit. It must be emphasized that the codes of practice are based upon empirical results; what actually happens in various kinds of molten metal/water explosions is imperfectly understood. However, attention to the codes of practice has ensured the virtual certainty of avoiding accidents in the event of “run-outs” with aluminum alloys.
In the last several years, there has been growing interest in light metal alloys containing lithium. Lithium makes the molten alloys more reactive. In the above mentioned article in “Metal Progress”, Long refers to previous work by H. M. Higgins who had reported on aluminum/water reactions for a number of alloys including Al—Li and concluded that “When the molten metals were dispersed in water in any way Al—Li alloy underwent a violent reaction.” It has also been announced by the Aluminum Association Inc. (of America) that there are particular hazards when casting such alloys by the DC process. The Aluminum Company of America has published video recordings of tests that demonstrate that such alloys can explode with great violence when mixed with water.
U.S. Pat. No. 4,651,804 teaches the use of the aforementioned casting pit, but with the provision of removing the water from the bottom of the cast pit such that no buildup of a pool of water in the pit occurs. This arrangement is their preferred methodology for casting Al—Li alloys. European Patent No. 0-150-922 describes a sloped pit bottom (preferably three percent to eight percent inclination gradient of the pit bottom) with accompanying off-set water collection reservoir, water pumps, and associated water level sensors to make sure water cannot collect in the cast pit, thus reducing the incidence of explosions from water and the Al—Li alloy having intimate contact. The ability to continuously remove the ingot coolant water from the pit such that a build-up of water cannot occur is critical to the success of the patent's teachings.
Other work has also demonstrated that the explosive forces associated with adding lithium to aluminum alloys can increase the nature of the explosive energy several times than for aluminum alloys without lithium. When molten aluminum alloys containing lithium come into contact with water, there is the rapid evolution of hydrogen, as the water dissociates to Li—OH and hydrogen ion (H+). U.S. Pat. No. 5,212,343 teaches the addition of aluminum, lithium (and other elements as well) with water to initiate explosive reactions. The exothermic reaction of these elements (particularly aluminum and lithium) in water produces large amounts of hydrogen gas, typically 14 cubic centimeters of hydrogen gas per one gram of aluminum −3% lithium alloy. Experimental verifications of this data can be found in the research carried out under US Department of Energy funded research contract number # DE-AC09-89SR18035. Note that claim 1 of the U.S. Pat. No. 5,212,343 claims the method to perform this intense interaction for producing a water explosion via the exothermic reaction. This patent describes a process wherein the addition of elements such as lithium results in a high energy of reaction per unit volume of materials. As described in U.S. Pat. Nos. 5,212,343 and 5,404,813, the addition of lithium (or some other chemically active element) promotes an explosion. These patents teach a process where an explosive reaction is a desirable outcome. These patents reinforce the explosiveness of the addition of lithium to the “bleed-out” or “run-out”, as compared to aluminum alloys without lithium.
Referring again to the U.S. Pat. No. 4,651,804, the two occurrences that result in explosions for conventional (non-lithium bearing) aluminum alloys are (1) conversion of water to steam and (2) the chemical reaction of molten aluminum and water. The addition of lithium to the aluminum alloy produces a third, even more acute explosive force, the exothermic reaction of water and the molten aluminum-lithium “bleed-out” or “run-out” producing hydrogen gas. Any time the molten Al—Li alloy comes into contact with water, the reaction will occur. Even when casting with minimum water levels in the casting pit, the water comes into contact with the molten metal during a “bleed-out” or “run-out”. This cannot be avoided, only reduced, since both components (water and molten metal) of the exothermic reaction will be present in the casting pit. Reducing the amount of water-to-aluminum contact will eliminate the first two explosive conditions, but the presence of lithium in the aluminum alloy will result in hydrogen evolution. If hydrogen gas concentrations are allowed to reach a critical mass and/or volume in the casting pit, explosions are likely to occur. The volume concentration of hydrogen gas required for triggering an explosion has been researched to be at a threshold level of 5% of volume of the total volume of the mixture of gases in a unit space. U.S. Pat. No. 4,188,884 describes making an underwater torpedo warhead, and recites page 4, column 2, line 33 referring to the drawings that a filler 32 of a material which is highly reactive with water, such as lithium is added. At column 1, line 25 of this same patent it is stated that large amounts of hydrogen gas are released by this reaction with water, producing a gas bubble with explosive suddenness.
U.S. Pat. No. 5,212,343 describes making an explosive reaction by mixing water with a number of elements and combinations, including Al and Li to produce large volumes of hydrogen containing gas. On page 7, column 3, it states “the reactive mixture is chosen that, upon reaction and contact with water, a large volume of hydrogen is produced from a relatively small volume of reactive mixture.” Same paragraph, lines 39 and 40 identify aluminum and lithium. On page 8, column 5, lines 21-23 show aluminum in combination with lithium. On page 11 of this same patent, column 11, lines 28-30 refer to a hydrogen gas explosion.
In another method of conducting DC casting, patents have been issued related to casting Al-LI alloys using an ingot coolant other than water to provide ingot cooling without the water-lithium reaction from a ‘bleed-out” or “run-out”. U.S. Pat. No. 4,593,745 describes using a halogenated hydrocarbon or halogenated alcohol as ingot coolant. U.S. Pat. Nos. 4,610,295; 4,709,740, and 4,724,887 describe the use of ethylene glycol as the ingot coolant. For this to work, the halogenated hydrocarbon (typically ethylene glycol) must be free of water and water vapor. This is a solution to the explosion hazard, but introduces strong fire hazard and is costly to implement and maintain. A fire suppression system will be required within the casting pit to contain potential glycol fires. To implement a glycol based ingot coolant system including a glycol handling system, a thermal oxidizer to de-hydrate the glycol, and the casting pit fire protection system generally costs on the order of $5 to $8 million dollars (in today's dollars). Casting with 100% glycol as a coolant also brings in another issue. The cooling capability of glycol or other halogenated hydrocarbons is different than that for water, and different casting practices as well as casting tooling are required to utilize this type of technology. Another disadvantage affiliated with using glycol as a straight coolant is that because glycol has a lower heat conductivity and surface heat transfer coefficient than water, the microstructure of the metal cast with 100% glycol as a coolant has coarser undesirable metallurgical constituents and exhibits higher amount of centerline shrinkage porosity in the cast product. Absence of finer microstructure and simultaneous presence of higher concentration of shrinkage porosity has a deleterious effect on the properties of the end products manufactured from such initial stock.
In yet another example of an attempt to reduce the explosion hazard in the casting of Al—Li alloys, U.S. Pat. No. 4,237,961, suggests removing water from the ingot during DC casting. In European Patent No. 0-183-563, a device is described for collecting the “break-out” or “run-out” molten metal during direct chill casting of aluminum alloys. Collecting the “break-out”or “run-out” molten metal would concentrate this mass of molten metal. This teaching cannot be used for Al—Li casting since it would create an artificial explosion condition where removal of the water would result in a pooling of the water as it is being collected for removal. During a “bleed-out” or “run-out” of the molten metal, the “bleed-out” material would also be concentrated in the pooled water area. As taught in U.S. Pat. No. 5,212,343, this would be a preferred way to create a reactive water/Al—Li explosion.
Thus, numerous solutions have been proposed in the prior art for diminishing or minimizing the potential for explosions in the casting of Al—Li alloys. While each of these proposed solutions has provided an additional safeguard in such operations, none has proven to be entirely safe or commercially cost effective.
Thus, there remains a need for safer, less maintenance prone and more cost effective apparatus and processes for casting Al—Li alloys that will simultaneously produce a higher quality of the cast product.